Glucose and sorbitol rapidly enter the cells of the heart of the 5-day chicken embryo. Glucose enters adult heart cells much more slowly and sorbitol does not enter adult cells at all. Two possible explanations of these results are: (a) The cell membranes of 5-day chick heart cells are highly permeable to glucose and sorbitol which enter by simple diffusion, (b) The cell membranes of 5-day heart cells possess transport systems of high capacity for glucose and sorbitol. The proposed research will distinguish between explanations (a) and (b). If glucose or sorbitol entry has the properties of carrier-mediated transport (i.e., stereospecificity, saturation kinetics, competition, counterflow) the specificities and kinetic parameters of the transport systems will be defined and the time course of the development of transport properties characteristic of adult cells will be observed. If glucose or sorbitol enters 5-day cells by simple diffusion, the lipid and protein composition of the plasma membranes of 5-day cells will be compared to the composition of membranes from later stages of development in the hopes of discovering chemical and structural bases for such divergent permeability characteristics. The embryonic development of insulin receptor sites in the heart cell plasma membrane and ability of insulin to stimulate the transport and phosphorylation of sugars will be studied.